This invention relates to polycrystalline cubic boron nitride, and more particularly, to a direct conversion process for making cubic boron nitride from coated, substantially oxide-free hexagonal boron nitride, and abrasive particles and articles made therefrom.
In the preparation and use of polycrystalline cubic boron nitride (CBN) prepared by the conversion of hexagonal boron nitride (HBN) to CBN, it has been found that the presence of oxide contaminants in the starting HBN interferes with the quality of the converted polycrystalline CBN and the sintering of the resulting converted polycrystalline CBN mass. In many of the current methods which have been adapted to remove the oxide contamination in the starting HBN, the surface of the HBN re-oxidizes before and during the process for the conversion of the HBN to CBN.
Another problem relating to the preparation and use, in for example, grinding wheels and cutting tools, of polycrystalline CBN prepared by conversion of HBN to CBN, is the control of the breakdown characteristics of the resultant directly-converted polycrystalline CBN particles during metal removal applications. A major factor in determining the performance of abrasive grinding wheels and cutting tools made from directly-converted polycrystalline HBN particles, is how the abrasive particles break down during the grinding application.
Cubic boron nitride (CBN), soft graphite (hexagonal) boron nitride (HBN) and other forms of boron nitride are described by Corrigan in U.S. Pat. No. 4,188,194 which is incorporated herein by reference in its entirety. In U.S. Pat. No. 4,188,194 hexagonal boron nitride (HBN) substantially free of catalytically active materials is maintained at pressures in excess of 60Kbar and temperatures of about 1800.degree. C. and higher for a period of time sufficient to directly convert the HBN to polycrystalline CBN. Corrigan ('194) discusses the detrimental effects of oxide contamination, for example, B.sub.2 O.sub.3, MgO and Al.sub.2 O.sub.3, and suggests as examples, the use of tantalum, titanium, vanadium and other Group IV metals; zirconium, molybdenum and other Group V metals; and hafmium, tungsten and other Group VI metals, as metals which do not interfere with the conversion/sintering process, yet prevent impurity penetration into the sample when used as shielding material.
In U.S. Pat. No. 4,289,503, incorporated herein by reference in its entirety, Corrigan makes cubic boron nitride from hexagonal boron nitride powder by removing boric oxide from the surface of the HBN and converting the HBN to CBN, in the absence of impurities which interfere with the conversion to CBN, by high pressure-high temperature treatment at 55-80 kilobars and from 1600.degree. C. to the reconversion temperature for a time sufficient to convert the HBN to CBN and sinter the CBN. The preferred HBN in Corrigan ('503) is pyrolytic boron nitride (PBN), and Corrigan utilizes vacuum heating or firing of the HBN powder to remove volatile impurities, particularly boron nitride surface contamination. Corrigan ('503) also discusses the mixing of graphite with HBN powder to prevent particle fusion. Corrigan, in U.S. Pat. No. 4,289,503, provides a coating of boron on the surfaces of the oxide-free HBN before conversion to CBN.
In one prior art method, CBN particles are coated with a metal, the metal being capable of forming a chemical bond with the particles, including the steps of providing a mass of metal for the coating in powdered form in contact with the particles, heat treating the metal powder and the particles at a temperature below the melting point of the metal to deposit a layer of metal on the particles and recovering the particles as discrete, metal coated particles, the heat treatment taking place in a non-oxidizing atmosphere and being chosen to allow chemical bond formation between the particles and the coating as described in U.S. Pat. No. 4,399,167. In the description of prior art set forth in U.S. Pat. No. 4,399,167, it is indicated that the metal coating of particles of CBN may be achieved by a variety of methods depending on the nature of the metal coating, the coating being applied electrolytically, electrolessly or by vacuum deposition, it being indicated that in the case of carbide formers, the most practical method of coating the particles of CBN being that of vacuum deposition. The preferred metals of U.S. Pat. No. 4,399,167 for coating CBN particles are titanium, manganese, chromium, vanadium, tungsten, molybdenum and niobium.
It can be seen from the foregoing, that it would be advantageous to form polycrystalline CBN from HBN by techniques which more completely remove the oxide contaminants, that is, which form a substantially oxide-free surface, and which prevent the formation of oxide contaminants, that is, prevent re-oxidation of the surface, during the direct conversion of HBN to CBN. More specifically, it would be advantageous to use HBN particles as a starting material wherein not only has the oxide been substantially removed from the surface of the HBN but also wherein the surface has been protected from re-oxidation before and/or during the direct conversion process to polycrystalline CBN. Furthermore, it would be advantageous to utilize a process to maintain the oxide-free HBN particles in an oxide-free state prior to and during the direct conversion to polycrystalline CBN by using a process and materials which do not interfere with the direct conversion process itself.
In order to improve the properties and other characteristics of polycrystalline CBN particles resulting from the direct conversion of HBN particles, it would be advantageous to control particle breakdown characteristics and to improve the control of the particles size while improving chemical bonding or interparticle bonding.